The Third Generation Project Partnership (3GPP) has developed the System Architecture Evolution (SAE) as the core network architecture of its future and Long Term Evolution (LTE) wireless mobile telecommunications standard. The main component of the SAE architecture is the Evolved Packet Core (EPC; see “Architecture enhancements for non-3GPP Accesses,” 3GPP TS 23.402). The LTE/SAE network includes network entities supporting the user and control planes.
An ongoing trend within telecommunications is the convergence of fixed and mobile networks, which is known as Fixed Mobile Convergence (FMC). The trend of evolving networks using IP-based technologies is common for fixed and mobile networks, which makes the convergence easier. By FMC, mobile and fixed network operators will be able to utilize their network resource more efficiently, which leads to reduction of capital and operational expenditure (CAPEX and OPEX). For instance, when a user is running an IP-based application such as Multimedia Telephony (MMTel) inside their home, it is more efficient to utilize broadband connectivity of the fixed access network rather than the wireless access network.
Residential networks have been important to the success of FMC because they are the most commonly used fixed network access by ordinary users. Therefore, it is important to be able to connect mobile phones to the EPC through a residential network. The term User Equipment (UE) can be used in place of the term mobile terminal or mobile phone. The term UE is familiar in the 3GPP documentation, and is intended to refer to any piece of equipment that is configured to access the internet; it would include, for example and without limitation, mobile telecommunication devices, portable or handheld computing devices and desktop or installed computers.
3GPP defines mobile 2G/3G/LTE accesses and “non-3GPP accesses” (TS 23.402). The latter can be a fixed network. The BBF (BroadBand Forum, the standardization organization for the fixed access; see http://www.broadband-forum.org/) defines an architecture for fixed networks. Many UEs address the FMC trend by providing multiple radio interfaces: one interface to connect to a 2G/3G/LTE access and a WiFi interface to connect to a fixed network via a Wireless Local Area Network (WLAN). FIG. 1 shows a simplified network architecture in which a UE 1 attaches to an Access Point (AP) 2 in a Radio Access Network (RAN) 3. In this example, the RAN 3 is a WLAN. The UE 1 can then access services from a service network 4 via a Core Network 5.
Some of the key drivers for the integration of WLAN with 3GPP are:    a) The large growth in mobile broadband traffic. To accommodate this, unlicensed WLAN spectrum can serve as a complement to the 3GPP Radio Access Technology (RAT) spectrum;    b) The wide support of WLAN connectivity in devices. Most modern mobile devices include both 3GPP radio and WLAN radio; and    c) The desire from operators to support the same services regardless access.
SaMOG (“Study on S2a Mobility based on GTP & WLAN access to EPC”) is, at the time of writing, a 3GPP work item. The aim of SaMOG is to allow a UE (User Equipment, in most cases a mobile device) to gain access to the 3GPP Evolved Packet Core (EPC) using WLAN as access technology. Currently, 3GPP Rel-11 does not support handover with IP address preservation between 3GPP and WLAN. Furthermore, a UE is restricted to having only a single Packet Data Network (PDN) connection or a single IP offload connection via WLAN. A PDN connection can be seen as a logical IP tunnel between the UE and the EPC network. An operator may also decide to offload the EPC. In this case, an IP offload connection is setup. The UE's traffic is then not routed via EPC, but directly offloaded to the Internet from the WLAN access network
SaMOG working items include the proposal to support IP address preservation upon a handover between 3GPP and WLAN. A “multi-PDN scenario”, includes not only support for handover with IP address preservation, but also support for multiple PDN connections via WLAN, and support for having one or more PDN connection via WLAN simultaneous with one or more IP offload connections via WLAN.
FIG. 2 shows a UE 1, an Access Point (AP) 2, an evolved NodeB (eNB) 6, a Trusted Non-3GPP Access Network Gateway (TWAG) 7, a Serving Gateway (SGW) 8, a PDN Gateway (PGW) 9 and an Authentication, Authorization, and Accounting (AAA) Server 10. These nodes and functions are described in more detail in 3GPP TS 23.402.
In step S1, the UE 1 associates with the AP 2. An authentication process S2 takes place. The TWAG 7 then sends a create session request S3 to the PGW 9, which responds S4. Step S5 provides an IP address to the UE 1, and step S10 indicates that the PDN connection between the UE 1 and the PGW 9 is established.
Note that FIG. 2 illustrates a General Packet Radio Service Tunneling Protocol (GTP) option. A similar process may be used in a Proxy Mobile IP (PMIP) network.
FIG. 3 is a signalling diagram in a multi-PDN scenario, and is a simplified version of a signalling diagram from SaMOG. In this example the first connection is an offload IP connection. Attachment parameters for the first connection are sent as part of authentication in step S12. A second connection, a PDN connection in this example, is setup in steps S16 to S22
SaMOG defines how a UE 1 attaches to the network and in particular how a PDN connection is setup via WLAN. It does not specify which AP 2 the UE 1 attaches to. Neither does it specify under which conditions the UE 1 can attach to a specific AP 2. On a high level, there are two ways to control a UE when to attach, and to which AP:
1. The first method is based on policies in the UE 1. These policies may be pre-configured in the UE 1, or may be downloaded from a network node. In a 3GPP architecture, such a network node is called Access Network Discovery and Selection Function (ANDSF). A policy rule may, for example, say “Attach to SSIDx when it is available”. This may be further refined to include performance measurements, for example “Attach to SSIDx only when the load of the AP is below a certain threshold”.2. The second method allows the network to decide when and where the UE 1 attaches. It then instructs the UE 1 to do so by an explicit command. This way, policies are kept inside the network. The network policy function may make an attachment decision based on measurements performed by the UE 1. For example, the UE 1 may be attached to a Long Term Evolution (LTE) network. The network then instructs the UE 1 to take measurements on connections with various WLAN APs. After receiving the measurement results, the policy function decides which AP 2 the UE 1 shall attach to. Finally, the policy function explicitly instructs the UE 2 to attach.